1. Field of the Invention
The present invention relates generally to the tooth structure of earth boring bits of the rolling cutter variety and, more specifically, to manufacturing techniques which add toughness to the areas of the teeth of steel-tooth bits which are most prone to cracking.
2. Description of the Prior Art
The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention that made that success possible. Only soft formations could be commercially penetrated with the earlier drag bit, but the original rolling-cone rock bit invented by Howard R. Hughes, U.S. Pat. No. 930,759, drilled the hard cap rock at the Spindletop Field, near Beaumont, Texas, with relative ease. Over the years, many individual advances have contributed to the impressive overall improvement in the performance of rock bits.
The early rolling-cone earth boring bits had teeth formed integrally with the cutters. These bits, commonly known as xe2x80x9csteel-toothxe2x80x9d or xe2x80x9cmill-toothxe2x80x9d bits, are still in common usage for penetrating relatively soft formations. The strength and fracture-toughness of the steel teeth permits relatively long teeth with long crests, which provide the aggressive gouging and scraping action that is advantageous for the rapid penetration of relatively soft formations.
However, it is rare that a formation interval will consist entirely of soft material with low compressive strength. Often, there are streaks of hard or abrasive materials that a steel-tooth bit must be able to penetrate economically, without damage to the bit.
Despite improvements which have occurred over the years, attention has again been recently focused on the development of earth-boring bits of the mill or steel-tooth variety because of advances in bearing and seal technology. During the manufacture of a typical steel-toothed cone for a rotary rock bit, the cone is first forged and the teeth are then machined in the desired shape and location. Next, the selected surfaces of each tooth are typically hard-faced. The wear-resistant materials or hardfacings which are utilized are conventional, and typically consist of particles of tungsten carbide or other hard metal dispersed in a steel or cobalt binder matrix. Such hardfacing materials are applied by melting the binder of the hardfacing material and applying the material over the surfaces of the tooth. Thereafter, the cutter is carburized to create a desired case depth. The cutter is then hardened and tempered.
Three rotatable cutters, so treated, are then typically assembled with the head section to form an earth boring drill bit. Steel teeth thus formed and subjected to a carburizing treatment develop a carbon penetration along the surface exposed to the treatment. The carbon penetrates into the steel and, it has been found, that such treatment tends to form an excess of carbon along the sharp corners, boundary lines and edges of the teeth. This undesirable concentration of carbides tends to make the teeth more brittle and more subject to fracture than the remainder the tooth surface. Cracking and breakage of the teeth of steel-tooth bits can thus be a problem, especially with the increased wear resistance of the hardfacing deposits, coupled with the longer service life of the bit.
A need exists, therefore, for an earth boring bit having a steel-tooth structure with improved toughness, particularly in the sharp corners of the teeth, near the base of each tooth.
It is, therefore, a general object of the present invention to provide an improved earth boring bit having an improved steel-tooth structure with added toughness in the area of each tooth which is most prone to cracking and brittleness.
This and other objects of the present invention are achieved by providing an earth boring bit having a bit body and at least one cutter rotatably secured to the bit body. The cutter has a plurality of teeth integrally formed thereon and arranged in circumferential rows. Each of the teeth includes a pair of ends, a pair of flanks, a crest transversely connecting the ends and flanks and a root, the roots of adjacent teeth being joined by a connecting region which defines a relatively sharp corner for the teeth adjacent the root thereof. The cutter has a generally uniform carburized case provided by a carburizing treatment during manufacture except at the sharp corners defined by the intersection of the flanks and ends of selected adjacent teeth of the cutter.
In the method of manufacturing the improved earth boring bit of the invention, a cutter body is formed having a plurality of teeth, each of the teeth having a pair of ends, a pair of opposing flanks, a crest transversely connecting the ends and flanks and a root, the roots of adjacent teeth being joined by a connecting region which defines a relatively sharp corner for the teeth. A temporary protective coating is applied to at least the sharp corners of selected teeth of the cutter. The temporary protective coating is effective to prevent the buildup of excess carbon in the sharp corner region of the selected teeth in a subsequent carburizing step. The cutter is then carburized to produce a carburized case therein, heat treated and mounted on a bit body.
The method can also include the step of applying a wear resistant material over the crest and a portion of the flanks and ends of at least selected teeth, the regions at the intersection of the flanks and ends of at least selected teeth being void of the wear resistant material. Preferably, the temporary protective coating is provided by a case preventative paint. The preferred wear resistant material is a tube hardfacing material. The case preventative paint can also be applied to the sharp corners and at least partly up a portion of the flanks and ends of adjacent teeth of the cutter prior to carburization.
Other objects, features and advantages of the present invention will become apparent to those having skill in the art with reference to the drawings and detailed description, which follow.